During the past three decades, there have been many efforts to reduce the level of sulfur content of fossil fuels. In the combustion process of an engine, the combustion of sulfur or sulfur containing compounds leads to sulfur dioxide formation. Sulfur dioxide can cause the formation of sulfate aerosol particles. Moreover, sulfur dioxide can form acidic rain causing adverse effects on the environment and artifacts. Fuel combustion processes are thus the main sources of sulfur dioxide emissions. Therefore, the amount of sulfur present in fuels correlates proportionally with the amount of sulfur dioxide emitted. The United States Environmental Protection Agency U.S.E.P.A has established several stringent regulations for sulfur content in road transportation fuels and non-road fossil fuels in order to control the emission of sulfur dioxide from the combustion of fuel.
Sulfur can be found in many different forms in fuel, but organic sulfides, thiophenes, benzothiophenes, and dibenzothiophenes are the major sulfur-containing compounds present in liquid fuels. Importantly, more than 85% of the sulfur-containing compounds in diesel fuel are thiophenes, and above 70% of the thiophenic compounds are benzothiophene (BT) and dibenzothiophene (DBT). Several methods including hydrodesulfurization (HDS), biodesulfurization (BDS), and oxidesulfurization (ODS) have been tested for fuel desulfurization.
The hydrodesulfurization method has limitations such as the usage of high concentration of hydrogen under extreme conditions of pressure and temperature over sulfided Co—Mo/Al2O3 and Ni—Mo/Al2O3. Furthermore, it provides low efficiency of removing sulfur from polycyclic aromatic sulfur heterocyclic compounds (PASHs), particularly dibenzothiophene (DBT) and its derivatives such as 4-methyl dibenzothiophene (4-MDBT), and 4,6-dimethyl dibenzothiophene (4,6-DMDBT). Even though the efficiency of the HDS process can be enhanced by increasing the severity of the HDS process conditions, there are undesirable side reactions which become dominant as the severity is increased. For instance, during the hydrodesulfurization of gasoline at high pressure, many olefins are saturated and the octane number decreases. Higher temperature also leads to increased coke formation and subsequent catalyst deactivation. These limitations cause a low effectiveness of the HDS method.
In the BDS method, the desulfurization process is achieved by using biochemical, microbiological, or enzymatic means to conduct the desulfurization. This method is performed under mild conditions and has shown higher selectivity than the HDS method. However, only a few enzymes have demonstrated the capability of removing sulfur atoms from heterocyclic molecules in fuel without causing oxidative loss of carbon.
The ODS method is based on the oxidation of thiophenic compounds to their corresponding oxidized forms FIG. 2. The physical and chemical properties of sulfoxides and sulfones are different from the corresponding non-oxidized forms; therefore they can then be separated easily from the body of fuel by extraction with polar solvents or by adsorption on silica gel. The ODS method has high efficiency of reducing the sulfur content in diesel fuel as well as that of nitrogen compounds under mild operating conditions, with no need of hydrogen gas. Significantly, the ODS process shows high reactivity with aromatic sulfur compounds. The oxidesulfurization method has a high efficiency when it is combined with other separation methods like adsorption or extraction. The combination of oxidation with simultaneous extraction of oxidized aromatic sulfur compounds from a non-polar phase to a more polar phase, was found to improve the oxidation process. The main idea of this technique was based on the oxidation of aromatic sulfur compounds to their more polar oxidized forms, followed by concentrating the more polar compounds into a polar solvent.